a simple google search would have answered your query but here you go: http://en.wikipedia.org/wiki/Search_engine_optimization SEO started back in 1997
according to me seo work start properly after 2000, but that time seo very easy, that time if you target any keywords sure you on first page with in 10-20 days but now your keywords some competitive sure you on first page after 10 to 20 months. so now seo very hard because daily increase more site, more competitive. so if you like on first page then sure you increase natural cum quality links. thank dikshawebsolutions
i started back linking, commenting on high page rank websites, doing forums and some bookmarking on social sites.
Not quite 1997, SEO exist even before that, probably can't be called exactly SEO, but Internet marketing. Before the time of "proper" search engines, there was a strategy to buy domain names which start with number, as search engines (more like directories) were sorting results alphabetically.
It has been around early 90's even before stated ' 1997 ' If given that it was used on different times. As others have stated Back linking stated prodominently early on and this system eventually grew to what is now.